1. Field of the Invention
The present invention relates to the manufacture of electronic modules, and more particularly to the soldering of connecting pins to conductor lands defined upon a surface of a circuitized dielectric substrate, typically comprised of a ceramic material. The pins provide electrical and mechanical connections to the outside world, and providing these connections absolutely reliably is an ongoing problem in the integrated circuit packaging art. The method and apparatus of the invention are particularly suitable for use in highly integrated, high density applications.
Most particularly, the present invention relates to a fluxless method and apparatus for providing precision solder connections which does not involve the use of pyrollidones, chlorofluorocarbons, perchloroethylene, or other halogenated solvents or any other chemicals the use of which is now considered to be environmentally inadvisable.
2. Background Art
Circuitized ceramic substrates are mounted to printed circuit boards or cards (PCBs) in order to enhance the electronic capabilities of the package. Typically, connector pins are positioned in holes formed in the ceramic at selected termination areas called lands on the metallized, circuitized "top" surface of the ceramic substrate. The head of each connector pin is soldered to a land in order to make reliable mechanical and electrical contact. The tail end of the connector pin emerges from the unmetallized, "bottom" surface of the ceramic substrate in order to make contact with a board or card or with another circuitized ceramic substrate.
Soldering establishes a bond between a solderable base material, typically, but not limited to, copper or gold plated copper, and a solder material, typically, but not limited to, a Pb-Sn alloy. In order to form a bond, the solder must wet the base material. In the manufacture of pinned substrates, the solder must wet both the head of each pin and the circuit pad to which it is intended to be soldered. If, as has been known in the art to happen from time to time, wetting is incomplete, the melted and hardened solder overhanging the pin head will leave a gap in which impurities can condense, and the resultant electrical/mechanical connection will be both discontinuous and unamenable to discovery by nondestructive microscopic examination techniques.
The process normally used to solder the lands of metallized ceramic substrates to pin heads involves use of a flux. Whereas flux is useful in many soldering applications, its use in high density packaging applications is limited. Components of the flux which remain within the solder, such as abietic acid from the flux resin, tend to "spit" flux components and solder balls under temperatures encountered during subsequent processing, a phenomenon which can cause solder shorting between closely spaced lands and lines. Flux may become trapped under the head of the connecting pin, interfering with the integrity of the connection.
The use of flux in a soldering process has been known in the art for many years. Flux serves the purpose of promoting adhesion between solder and metal soldered. If the metal to be soldered tends to form an oxidized surface layer in air, as does copper, the flux provides a cleansing medium. Different ways to solder using an aligning plate are known in the art. For example, U.S. Pat. No. 4,871,110, issued Oct. 3, 1989 to Fukasawa et al describes a duplex arrangement of aligning plates, the bottom of which employs vacuum to position solder balls onto holes, which technique is intended to overcome the problem of solder balls adhering to one another due to static electricity. The alignment plates themselves are glass or ceramic, the diameter of the vacuum holes in the bottom alignment plate being smaller than the solder balls and the diameter of holes in the top alignment plate being larger than the solder balls. There is no mention of the elimination of liquid fluxing chemicals, however, and the duplex fixturing plus vacuum is more complex and quite different from the technique of the present invention.
Because of the cleansing properties of the flux, it is not necessary to reflow the solder in a reducing gas even when gas reflow of solder is used, as described in IBM Technical Disclosure Bulletin Vol. 20, No. 2, published July, 1977 by Rivenburgh et al, which describes the use of a vacuum-vibration solder plate fixture which is made of anodized aluminum, stainless steel or graphite, materials described as being non-wettable by the solder. The fixture described restricts solder movement during reflow, functioning more in the nature of a solder mask than a positioning plate, and the use of liquid flux is also described.
U.S. Pat. No. 4,462,534 issued Jul. 31, 1984 to Bitaillou et al. is assigned to the same assignee as is the present invention. The '534 patent describes the positioning of solder balls on the heads of connecting pins on a ceramic substrate using liquid flux without an alignment plate. The '534 patent describes a technique which the present invention, a reducing technique in combination with an alignment plate, is intended to replace.
Likewise Research Disclosure, June, 1989, Number 302, published anonymously, describes using solder balls and liquid flux. A plate fixture is also used, but not for positioning the solder balls, merely for restraining the solder balls from moving.
Fluxless processes for applying solder balls are also described in the art. For example, U.S. Pat. No. 3,719,981 issued Mar. 13, 1973 to Steitz describes positioning of solder balls in the absence of flux by arraying the solder balls on the tacky surface of a piece of pressure sensitive tape, such as "Scotch" brand, and a combination of masks. The tape is removed after solder reflow.
IBM Technical Disclosure Bulletin Vol. 30, No. 7, published December, 1987 by a present inventor, describes a heat dissipatable flux or wax membrane for positioning solder balls.
Of course, neither of these processes is applicable to the high density, solvent free application of the present invention in which processing conditions and demands for cleanliness do not permit the use of glued cellulose tape or wax.
U.S. Pat. No. 4,632,295 issued Dec. 30, 1986 to Brusic et al and 4,919,729 issued Apr. 24, 1990 to Elmgren et al are useful for their descriptions of furnace soldering in a reducing atmosphere. The former includes an application step of dehydric or polyhydric alcohol, however, and the latter teaches away from the use of solder preforms and templets when light dimension tolerances are required, in contrast to the present invention.
Western Electrical Technical Digest No. 2, published April, 1966 by Costello describes the use of an anodized aluminum fixture for soldering but the fixture functions as a sleeve for the pin, the seam of which is to be soldered rather than the head, unlike the present invention.
Any flux residues must be removed subsequent to soldering. The solvents used for flux residue removal include halogenated organics, pyrollidones and other chlorofluorocarbons. It would be helpful to eliminate these solvents from the solder process, as they have been identified as being environmentally inadvisable. While liquid solder flux in general is not a fixed composition, commonly found ingredients also include abietic acid and other pine derivatives. These materials may remain in the solder and erupt, or "spit" suddenly during times when the solder is reheated, potentially contaminating high density electronic devices. Perchlorethylene, methylene chloride or Freon type solvent rinsing is typically used to remove flux residue. These solvents tend to leave behind difficult-to-remove halogen ions, and the used rinse requires special handling, typically involving distillation, tank farms or trucking, in order to minimize release to the environment. Some emissions do escape to the atmosphere after treatment in a carbon absorption unit.
It is also known to solder dip all or part of a circuitized substrate. For grossly dimensioned applications, dipping may result in an acceptable product, as gaps where solder does not wet may not make a practical difference. However, for both grossly dimensioned and high density applications, the present method and apparatus, wherein the solder is directed and limited to the exact places where it is needed and wherein wetting is complete, reliable and reproducible, is effective. The method and apparatus of the invention are simple, replacing a costly S.T.E.P. (Substrate Tinning Enhancement Program) machine, which requires debugging and has been used for soldering in 100 mil pin grid devices but not for interstitial and denser pin grids.
It has been demonstrated, in connection with 50 ml grid arrays, that the present invention is useful for soldering about one million solder balls in a six hour period.
Nothing in the art describes the fixture of the present invention in combination with a reducing gas soldering atmosphere, which permits precision soldering, thereby obviating the need for chlorofluorocarbon or other organic solvent.